The present invention generally relates to a diversity communication method in a mobile communication system among a radio base station and mobile transceivers used in time-division time-compressed multiplexing telephone or data signals for radio communication channels (hereinafter referred to as a "TCM communication system"). More specifically, the present invention is directed to a diversity communication method for a TCM communication system in which a certain radio channel is given among a number of radio channels provided for the communication system. If one mobile transceiver among a large quantity of mobile transceivers within a service area establishes communication with a corresponding radio base station using a given radio channel, time slot in each frame and communication signal by setting a radio communication line therebetween by way of, for instance, a telephone signal, and if another mobile transceiver attempts to establish communication by utilizing the same radio channel, but different time slot in the frame, an independent radio communication line is set up using the same radio channel but different time slot in the frame between the last-mentioned mobile transceiver and the above-described radio base station without causing an adverse influence on the communication already established between the first-mentioned mobile transceiver and the radio base station. As a result, various signals such as telephone signals, facsimile signals and data signals can be communicated through the same radio communication channel. In mobile communication systems using voice transmission to which a cellular system has been applied, the method for utilizing a multiplex signal which has been time-divided and time-compressed has been described in the following publications:
Publication No. 1: S. ITO, "System Design of Portable Telephones,--A Proposal for Time-division/Time-compression FM mobile Radio System--", Technical Report of "Institute of Electronics, Information and Communication Engineers", RCS 89-11, July 1990.
Publication No. 2: S. ITO "System Design of Portable Telephones,--Theoretical Analysis of Multipath Propagation characteristics of Time-division, Time-compression Multiplexing FM Mobile Radio System--", Technical Report of "Institute of Electronics, Information and Communication Engineers", RCS 89-47, January 1991.
Publication No. 3: S. ITO "Theoretical Analysis of Non-Simultaneous Load Advantage of time-division, time-compression multiplexing telephone signal, and Application to FM mobile System", Technical Report of Institute of Electronics, Information and Communication Engineers, RCS 89-65, March 1992.
In publication No. 1, the following system has been reported. A transmission signal (baseband signal) is segmented using a predetermined time interval, and then the segmented baseband signals are stored in a storage circuit. When these signals are to be read out, they are read out to a predetermined time slot at a speed "n" times higher than the storage speed. A carrier wave is angle-modulated or amplitude-modulated by the signals contained in this time slot and radio receiving circuits each having a receiving mixer, radio transmitting circuits each having a transmitting mixer, and switching circuits provided for a synthesizer applied to the receiving mixer of the radio receiving circuit and also for a synthesizer applied to the transmitting mixer of the radio transmitting circuit are employed and built into the mobile transceiver and the radio base station in order to send/receive the signal in a temporal interrupt mode. The outputs of these synthesizers are interrupted, where this interrupt state is synchronized for both transmission and reception. Also, the method for synchronizing the interrupt transmission/reception is also applied to the radio base station communicated with the radio transceiver. At the reception side, to derive only the signals stored in the above-described time slot, the radio receiving circuit is opened/closed so as to receive the signals, and the modulated signals are stored in the storage circuit so that when the stored signals are read out therefrom, the baseband signals corresponding to the transmitted original signal can be reproduced by reading out these signals at a low speed equal to 1/n times the storing speed.
Also, in publication No. 2, there have been proposed considerations of adverse influences in multi-path fading caused by the TCM signal during space transmission, and a guard time is set between the time slots in order to reduce or mitigate this adverse influence.
Furthermore, in publication No. 3, it is revealed that a multiplex load gain similar to that of the conventional FDM (frequency division multiplex system) signal is present in the time division/time compression multiplex system (TCM system), and also the quantitative idea and the system operation thereof are described. Then, when this multiplex load gain is used with deep FM modulation, the transmission power can be considerably lowered, and thus a considerable power saving may be achieved in the mobile transceivers.
However, there is no description with respect to the signal transmission in these conventional systems, concerning methods to overcome the adverse influences caused by fading which occurs when the signals are transmitted through the transmission medium, especially a space such as the transmission diversity. Also, in such a communication system where the radio communication line is set by the radio base station and the corresponding mobile transceiver is set using the telephone signal, there is no discussion in any of these publications of a case where the same carrier wave is hybrid-modulated by the same signal, i.e., in conventional systems, angle modulation and amplitude modulation are performed using two different signals. In other words, angle modulation on one signal and amplitude modulation on another signal, or amplitude modulation on the one signal and angle modulation on the other signal are performed. Thus, there exists a need for communication which is established using a hybrid-modulated wave which has advantages over the prior art systems.
In the conventional systems, when the radio communication lines are set by the radio base station and a large number of transceivers attempt to establish communication by utilizing, for instance, a TCM-processed telephone signal, in order to improve qualities of the signals to be transmitted, if a transmission diversity system (it is assumed that the multiplexing number is selected to be 2) is used, there are the following drawbacks in the utility of frequencies. First, the number of frequencies required in the frequency diversity system becomes two times larger than that required by a non-diversity type conventional system. Also, in a transmission diversity system (it is assumed that the number of multiplex is selected to be 2), even if the number of frequencies used is the same as that of a non-diversity system, the required transmission time will be two times longer than that of the non-diversity system. As a result, the effective ratio of the frequency is reduced to only one-half, a significant drawback in the art.